This often happens when a Gmail account is set up to "gather" or collect email from Outlook.com via POP or IMAP.
It's a quite handy feature as Gmail allows you to pull those messages and have a single unified "one true in-box" for all your email accounts. Coupled with the option of configuring "sending mail as another account" with true SMTP authentication and relaying, Gmail is able to act as a desktop email client as far as Outlook.com is concerned.
My guess is at some point you did configure at least the mail fetching part on a Gmail account. If you don't own a Google account, disable POP and IMAP access, and you're done.
If you do own a Gmail account, check the settings under "account preferences" where you can stop that mail fetching without disabling POP or IMAP on Outlook.com.